1. Field of the Invention
This invention relates to a catalyst for purifying an exhaust gas and an exhaust gas purifying system. More particularly, it relates to an exhaust gas purifying catalyst for simultaneously divesting the exhaust gas from the internal-combustion engine such as on an automobile of such noxious components as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) and an exhaust gas purifying system.
2. Description of the Prior Art
Various inventions have been proposed concerning exhaust gas purifying catalysts capable of removing noxious components from the exhaust gas which issues from the internal-combustion engine.
Heretofore, it has been universally known that palladium catalysts possess high heat-resistance and exhibit great purifying ability to the CO and HC in the oxidizing atmosphere [the so-called lean portion in which the air fraction is larger in the air/fuel mixture (A/F)] of the exhaust gas from the engine. They nevertheless have the disadvantage that they are deficient in the NOx purifying ability in the reducing atmosphere [the so-called rich portion in which the fuel fraction is larger in the air/fuel mixture (A/F)] of the engine exhaust gas. Thus, the palladium catalysts are used only on the lean side of the atmosphere in the form of a so-called oxidizing catalyst. Otherwise, they are used in the form of a three-way catalyst combining the palladium with rhodium, a substance possessing great NOx purifying ability, and effecting simultaneous removal of CO, HC, and NOx.
Since rhodium is very expensive, a desire has been expressed in the industry to decrease the amount of rhodium to be used as a catalyst component or avoid completely the use of rhodium as such. Since rhodium is characterized by possessing great NOx purifying ability, however, it constitutes itself an indispensable component for an exhaust gas purifying catalyst to be used for simultaneous removal of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx).
An object of this invention, therefore, is to provide a novel exhaust gas purifying catalyst and an exhaust gas purifying system.
Another object of this invention is to provide an exhaust gas purifying catalyst capable of simultaneously removing the three components, CO, HC, and NOx, at an extremely small application rate without requiring use of rhodium and an exhaust gas purifying system.